1. Field of the Invention
The present invention relates to an anti-skid control system for controlling the braking force applied to road wheels in a braking operation of an automotive vehicle to prevent the road wheels from being locked, and more particularly to an anti-skid control system for appropriately controlling the braking force applied to driven wheels which are linked by a differential gear mechanism.
2. Description of the Prior Art
In order to prevent road wheels from being locked, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to each wheel brake cylinder operatively connected to each road wheel. In the anti-skid control system, as disclosed in the U.S. Pat. No. 5,016,178 for example, a rotational speed of each road wheel or wheel speed is detected, on the basis of which either one of a decrease mode, an increase mode and a hold mode is determined for each wheel brake cylinder, and the hydraulic braking pressure in each wheel brake cylinder is controlled in response to the determined mode, so as to obtain the maximum coefficient of friction.
In general, an ordinary passenger vehicle has a pair of road wheels at each of its front and rear sides. either the front road wheels or the rear road wheels are operatively connected with the internal combustion engine to be driven directly thereby, while the rest of the wheels are not connected with the engine so as to be non-driven wheels. A vehicle having the driven wheels at its front side is called a front drive vehicle, while a vehicle having the driven wheels at its rear side is called a rear drive vehicle. Further, there has been provided a differential gear mechanism for compensating a difference between the rotational speeds of the right and left driven wheels so as to ensure a smooth driving of the vehicle. In other words, the right and left driven wheels are controlled by means of the differential gear mechanism such that a total speed of the rotational speeds of the right and left driven wheels is maintained to be constant.
Especially, according to a vehicle having a manually shifted transmission, when the transmission is in such a condition that its gears are meshed for shifting, if the anti-skid operation begins and a large skid occurs on one of the driven wheels, the other one of the driven wheels will be restrained from skidding by means of the differential gear mechanism, so that the hydraulic braking pressure in the wheel cylinder (hereinafter, simply referred to as the wheel cylinder pressure) will be raised. Consequently, if one of the driven wheels is recovered from skidding so that an engine torque is equally applied to the right and left driven wheels, a larger skid will occur on the other one of the driven wheels, because the wheel cylinder pressure applied thereto has been raised to a high pressure. Thus, when the anti-skid control operation begins, the large skid occurs alternately between the right and left driven wheels to cause a vibration of a vehicle body, so that a brake feeling is deteriorated.
Hereinafter will be described the above-described vehicle vibration caused in the anti-skid control operation of the above-described prior art with reference to FIG. 7, which shows variation of a wheel speed Vwr of a driven wheel DR at the fore right side of a front drive vehicle for example, a wheel speed Vwf of a driven wheel DL at the fore left side of the vehicle, and wheel cylinder pressures Pwr, Pwf for the driven wheels DR, DL.
At the outset, when the skid occurs on the driven wheel DR at the fore right side, a large driving torque is transmitted to the driven wheel DL at the fore left side by means of the differential gear mechanism, so that the driven wheel DL is restrained from skidding and the wheel cylinder pressure Pwf continues to be increased. When the wheel cylinder pressure Pwr for the driven wheel DR is decreased at a position "a" in FIG.7, and the wheel speed Vwr of the driven wheel DR is recovered to cause the differential gear mechanism to operate, the wheel cylinder pressure Pwf for the driven wheel DL is so high that a large skid of the driven wheel DL occurs, and the decrease of the wheel cylinder pressure Pwf begins at a position "b". In other words, since the wheel cylinder pressure Pwf for the driven wheel DL is large during the period of the position "a" to the position "b", the driven wheel DL skids to a large extent, so that the wheel cylinder pressure Pwf begins to be decreased at the position "b". When the wheel cylinder pressure Pwf for the driven wheel DL is increased at a position "c" and the wheel speed Vwf begins to be recovered, the wheel speed Vwr of the driven wheel DR is decreased by means of the differential gear mechanism, and the wheel cylinder pressure Pwr begins to be decreased at a position "d". Thus, a large skid occurs alternately on the driven wheels DR, DL, so that the wheel speeds Vwr, Vwf vary as shown in FIG. 7.
In order to prevent the above-described vibration of the vehicle body from occurring, an electromagnetic clutch mechanism, for example, may be provided to shift the gears into a neutral position at the time when the anti-skid control begins. Or, a so-called idle-up apparatus may be provided to increase an idling speed of the engine at the time when the anti-skid control begins. However, the provision of an electromagnetic clutch mechanism, idle-up apparatus and the like necessarily result in an increase in cost.